This invention concerns a thermal printing device, and particularly an integrated circuit for driving the thermal printing device and an assembling method therefor.
A thermal printing device includes an alumina or ceramic insulating substrate coated with a glaze, a plurality of heat generating resistors arranged on the insulating substrate, and an integrated circuit for driving the heat generating resistors, whereby each bit of the integrated circuit (IC) is supplied with the processed digital pulses, so that each bit independently makes a switching operation according to the pulses so as to drive the heat generating resistor connected thereto. Then, the heat produced by the current flowing through the heat generating resistor is transmitted to a heat-sensitive paper to blacken portions thereof, thus accomplishing the printing operation. Namely, the thermal printing device converts digital signals into an image by means of the heat produced from the heat generating resistor.
As the number of heat generating resistors per inch is increased, the resolution of the printing is improved. Conventionally, the thermal printing device with a high resolution such as 300 dpi (dots per inch; dots/inch) and 400 dpi may be obtained as follows:
First, a number of 64-bit bi-directional driving integrated circuits for 64-bit wire bonding in which the output pads are scatteredly arranged on both edge portions thereof longitudinally mounted on a substrate.
Second, instead of using the wire bonding to connect portions with gold wires, there are formed solder or gold bumps on the portions to be connected, in which the solder or gold bumps are connected to each other, which produces a flip or bump chip as a general driving integrated circuit.
Third, there is used a general driving integrated circuit exclusively using a tape automated bonding (T.A.B.) method which includes the steps of adhering a tape to the portions where gold wires are put by using solder or gold bump, arranging a thin wire on the central portion of the tape, and connecting the wires to each other.
All of the above three cases use the same method in forming heat generating resistors and signal distribution lines on an insulating substrate, but they use a different method in forming an electrical connection between a printed circuit board and a driving integrated circuit.
Referring to FIGS. 1A, 1B and 1C that illustrate conventional assembling methods, FIG. 1A shows the assembling method of using the wire bonding, wherein a thin gold wire 5 is used to connect semiconductor chip 1 and pad portion 3, Fig 1B shows the assembling method of using the flip chip, wherein a gold bump is used to connect substrate 7 and semiconductor chip 9, and FIG. 1C shows the assembling method of using the TAB method, wherein a tape 17 is used to connect semiconductor chip 13 and pad portion 15.
Comparing the three assembling methods, the wire bonding and the TAB method employ only the edge portions of the chip, so that the minimum pitch between the bonding pads limits increase of the resolution. Namely, the minimum pitch is respectively about 135 and 80 .mu.m for the wire bonding and TAB method. As the thermal printing device gains higher resolution, the pitch becomes smaller. Hence, if the pitch would be reduced below the minimum value, the bonding pads become adhered to each other. On the other hand, the flip chip has a minimum pitch of about 250 .mu.m greater than the minimum pitch obtained by the wire bonding and TAB method, but the bonding pads may be arranged on the whole surface of the chip so that the pattern of the signal distribution lines makes it possible to achieve a high resolution.
Referring to FIGS. 2A-2C for illustrating power source ground pads (GND pads) and internal signal distribution lines formed in a conventional driving integrated circuit, FIG. 2A shows the three GND pads 21 and the internal signal distribution lines 23 thereof formed in integrated circuit 20, FIG. 2B shows the five GND pads 25 and the internal signal distribution lines 27 thereof formed in integrated circuit 20, and FIG. 2C shows the nine GND pads 29 and the internal signal distribution lines 31 thereof formed in integrated circuit 20.
If the number of the bits of the driving integrated circuit shown in FIG. 2A is 128, the power source ground pads are respectively positioned on the three places indicating 1st bit, 64th bit and 128th bit. In this case, the values of the driving voltages applied to the bit positions forming the power source ground pads, i.e., 1st, 64th and 128th positions, considerably differ from the values of the driving voltages applied to the intermediate positions between the power source ground pads, i.e., 32nd and 96 bit positions due to great path differences.
Of course, although, as shown in FIGS. 2B and 2C, as the number of the power source ground pads 25, 29 is increased more than in the case of FIG. 2A, the voltage differences between the bit positions forming the power source ground pads and the intermediate positions are reduced, they still continue to have considerable values.
The drawbacks of the conventional assembling methods and of forming the power source ground pads are enumerated as follows:
First, in the wire bonding and the TAB method, since the minimum pitch is limited to a specific degree, the increase of the resolution can not be implemented easily.
Second, when assembling 64-bit bi-directional integrated circuit by using the wire bonding, the chip has to be mounted longitudinally on the substrate, so that the size of the product is increased.
Third, the manufacturing process is difficult, and the reliability of the product is hardly secured. Namely, when assembling the 64-bit bi-directional integrated circuit by using the wire bonding, there should be formed an insulating layer beneath the integrated circuit in order to insulate the substrate from the chip, which makes the process complicated. Moreover, when forming the pattern of the signal distribution lines, they are bent into L-shaped forms, and as the resolution is increased, the signal line distribution width is reduced, and also the bent portions are increased, thereby decreasing the reliability of the final product. When employing the flip chip method, since the solder or gold bump has to be melted by momentarily applied heat, which exerts a thermal shock to the device to possibly produce defects therein. Furthermore, since the solder or gold bump is to be arranged in a specified position, high precision technique is required. Also, the T. A. B. method needs a separate tape carrier film exclusively used, and complicates the process.
Fourth, when the power source ground pads are arranged on the edge portions of the integrated circuit in line with the other signal pads, the path differences of the internal signal distribution lines cause differences in the local printing concentration. Namely, the external digital signals of the integrated circuit inputted into the power source ground pads are transmitted through the internal signal distribution lines to the heat generating resistors. In this case, if, as shown in FIGS. 2A-2C, the power source ground pads are arranged on the edge portions of the integrated circuit in line with the other signal pads, there result in the path differences of the signal distribution lines in accordance with the number and positions of the power source ground pads on the integrated circuit, and thus the differences of the resistance values, which cause the differences of the driving voltages applied to the bits of the driving integrated circuit chip so as to effect the differences of the local printing concentration. These phenomena become more prevalent as the thermal printing device gets the higher resolution, i.e., the number of the driving bits is increased.